Austenitic alloy



United States Patent Ofifice 3,055,057 Patented Nov. 20, 1962 No Drawing. Filed Ian. 21, 1959, Ser. No. 783,028 12 Claims. (Cl. 75-124} This invention relates to austenitic iron, nickel, chromium alloys strengthened by the addition of molybdenum, vanadium, titanium, aluminum and boron and in particular to austenitic iron-base alloys having a critical combination of titanium, manganese and silicon.

Austenitic iron-base alloys, as precipitation hardened by titanium, have been known for quite some time. Attempts have been made to increase the mechanical properties of these alloys by utilizing higher amounts of titanium. During such prior attempts, it was found that in the normal air melting procedure a great portion of the titanium additions was lost, usually through the oxidation of the titanium into the form of titanium dioxide which was either trapped in the melt as an inclusion or floated out of the melt and into the slag. The entrapped inclusions resulted in the alloy being substantially unworkable. Considerable research has been done on the problem of increasing the concentration of titanium, and with the commerical advent of vacuum induction melting and consumable electrode electric arc controlled atmosphere melting, it was possible to make alloys containing controlled higher amounts of titanium. It was found, however, that the mere increase in the titanium content of these austenitic iron-base alloys did not appreciably increase the attainable mechanical properties. Metallographic examination revealed that excess phases were present when the alloys contained in excess of about 2% titanium.

Other investigators have found that in order to take advantage of the use of titanium as a precipitation hardening element in austenitic iron-base alloys it is necessary to form a strengthening precipitate, such as Ni Ti and Ni (Al, Ti). Investigations have also revealed that a large concentration of other non-strengthening precipitates forms in austenitic iron-base alloys containing in excess of about 2% titanium, and some of these phases contain a high concentration of titanium thus depleting the alloy of a portion of the titanium available for forming NigTi and Ni, (Al, Ti). Two of these titanium-rich phases have been identified by investigators and have been tentatively referred to as G-phase and M Ti. With increasing amounts of titanium present within the austenitic ironbase alloys, it has been observed that the concentration of these non-strengthening phases has correspondingly increased, especially where the titanium content is increased to amounts in excess of 2.0%. It therefore becomes apparent that increasing the titanium content alone is ineffective for increasing the mechanical properties of these alloys. When it is considered that it is necessary to use the vacuum melting or the consumable electrode controlled atmosphere melting processes which are more costly than the air melting process to produce the alloy containing a high concentration of titanium but that the resultant alloy does not exhibit improved mechanical prop erties, it will be appreciated that heretofore it was not possible to commercially produce this type of austenitic iron-base alloys containing in excess of 2.0% titanium.

It has been determined that the G-phase is composed of silicon, titanium and nickel in a stoichiometric ratio of approximately Ni Ti Si It appears in a microstructure as tiny globules and is formed at temperatures between 1500 F. and 1800 F. M Ti has been identified as Fe Ti with the elements silicon, manganese and to some degree chromium being substituted for iron in this phase. It is normally observed in the as-cast structure and when once removed, does not reappear unless the alloy is overheated to temperatures in excess of about 2275 F.

My investigation has provided basis for the belief that silicon promotes the formation of G-phase and manganese promotes the formation of M Ti. I have now found that increased mechanical properties can be obtained through the use of higher concentrations of titanium if a critical level of manganese and silicon content are maintained therein.

An object of this invention is to provide austenitic ironbase alloys having critical combination of titanium, manganese and silicon, which alloys are suitable for use under high stresses at elevated temperatures of up to at least 1300 F.

Another object of this invention is to provide an austenitic iron-base alloy containing nickel, chromium, molybdenum, vanadium, boron and aluminum as essential elements together with a critical combination of titanium, manganese and silicon as essential elements also.

A more specific object of the invention is to provide an austenitic iron-base alloy suitable for use at elevated temperatures of up to at least 1300 F. and which has improved properties resulting from a critical combination of titanium, manganese and silicon.

Other objects of this invention will become apparent from the following description:

The alloy of this invention is an austenitic iron-base alloy and comprises a composition which, within its broadest limits, includes between about 0.01% and about 0.15% carbon, from traces up to about 0.50% manganese, from about 0.05% silicon to about 0.80% silicon, from about 12.0% to about 22.0% chromium, from about 15.0% to about 35.0% nickel, from about 0.25% to 5.0% molybdenum, from about 2.0% to about. 6.0% titanium, from traces up to about 1.25% aluminum, from about 0.10% to about 1.5% vanadium, from about 0.001% to about 0.60% boron and the balance iron with not more than about 1.5% incidental impurities which may nor mally include phosphorus, sulfur, copper, cobalt tungsten and any other elements usually found and picked up as extraneous in the normal melting of austenitic iron-base alloys.

Reference may be had to Table I which illustrates the chemical composition of the general rangeand of the optimum range for obtaining the optimum combination of mechanical properties.

Element General range Optimum range 0. 015 Balance Each of the elements contained within the analysis of the alloy of this invention performs a specific function. Carbon in combination and cooperation with boron and titanium contributes materially to increasing the rupture ductility and the notch-rupture life of the alloy when each of the elements carbon, boron and titanium is properly proportioned within the alloy. At least 0.01% carbon is necessary from the standpoint of mechanical properties and austenite stability, whereas carbon contents in excess 3 of 0.15% make the alloy difiicult to fabricate and seriously impair machinability and corrosion resistance. Optimum results are obtained when the carbon content is maintained within the range between 0.01% and 0.08%.

While manganese is effective where the alloy is produced by the air melting method, its effect upon the mechanical properties and especially in the formation of M Ti in alloys containing more than 2% titanium is such that the manganese content must be severely limited. It is preferred to maintain the manganese content as low as possible; however, amounts of up to 0.50% can be tolerated without severely affecting the mechanical properties. Since it is not practical to melt the alloy without a pickup of manganese, optimum results are obtained Where the manganese content does not exceed 0.35%.

Silicon is present within the solid solution of the alloy and while it may contribute somewhat to the oxidation resistance of the alloy, its primary effect is that of conferring ductility on the alloy. In particular, it has been found that with lower silicon contents, that is, below about 0.05%, the alloy has a very low creep rate. However, the low creep rates are associated with the loss of ductility and a shortening of the rupture life of the alloy. It therefore becomes apparent that a small amount of silicon is necessary to impart the ductility to this alloy without adversely affecting the creep rate or rupture life. Accordingly, it has been found that a silicon content between about 0.05% and about 0.80% is necessary. Silicon in excess of about 0.80% adversely affects the creep rate of this alloy. Optimum results are obtained where the silicon content is maintained within the range between 0.35 and 0.70%. When the silicon content is maintained with in the optimum range, the optimum combination of creep rate, ductility and rupture life is obtained.

Chromium is the predominant element for providing corrosion resistance and oxidation resistance to the alloy when it is used at elevated temperatures. Chromium also enters the solid solution and contributes to the strength of the matrix when the chromium content is maintained within the general range. Chromium contents in excess of about 22% have a tendency to form intermetallic phases which, when present, reduce the room temperature tensile ductility and adversely affect the rupture strength. Although chromium is a strong ferrite-forming element when it is in solid solution at elevated temperatures, it reacts to retard structural changes and thus to stabilize the alloy. At least 12.0% chromium is necessary for adequate corrosion and oxidation resistance. It is preferred to maintain the chromium content within the range between 13.5% and 16.0% in order to obtain the optimum combination of chemical, physical and mechanical properties as respects the chromium content.

Nickel is the predominant austenitic-forming element and acts in cooperation with the chromium to provide sufficient oxidation and corrosion resistance. Nickel is also essential in that it reacts with titanium to form a precipitate identified as NigTl and this reaction is the major strengthening process occurring in the alloy when sufficient titanium is present. While cobalt can be substituted for a portion of the nickel in direct proportion to each other, it is preferred to maintain at least 15.0% nickel for the precipitation hardening process to: occur. While nickel contents in excess of about 35.0% can be utilized and replace the iron content, this materially increases the cost of the alloy and does not produce any outstanding increase in the mechanical characteristics of the alloy.

Molybdenum within the range between 0.25% and about 5.0% materially contributes to the strengthening of the solid solution, and it is particularly effective for offsetting the embrittling efiect which is normally expected with the addition of certain other alloying elements. At least 0.25% is necessary for any appreciable strengthening effect, whereas molybdenum contents in excess of about 5.0% have a tendency to decrease the ductility of the alloy. Optimum results appear to be obtained when the molybdenum is maintained Within the range between about 1.0% and about 1.5%. Molybdenum also contributes to the corrosion resistance of the alloy when it is used in an atmosphere containing the halogen ion.

Aluminum has been considered to contribute somewhat towards the embrittlement of this type of alloy. It has been found, however, that up to 1.25% aluminum can be utilized without any appreciable embrittlement occurring. Aluminum contents between about 0.10% and about 0.50% appear to aid the precipitation hardening reaction by cooperating with the nickel and the titanium .to form the precipitate Nlg (Al, Ti). The presence of aluminum also contributes somewhat to the oxidation resistance of this alloy.

Vanadium is present within the range between 0.10% and about 1.5% and enters into the solid solution of this alloy and contributes somewhat to the reduction of the embrittling effect encountered with the use of titanium and aluminum in this alloy.

Boron within the range between 0.001% and 0.060% is highly critical in that it is effective for greatly increasing the rupture ductility of the alloy. Of particular significance is the fact that at least 0.001% boron is necessary to suppress the formation of the cellular precipitate associated with these alloys. While boron contents in excess of about 0.060% do not appear to produce any substantial increase in the mechanical properties, it has been found that the higher boron concentrations together with the higher titanium contents present within this alloy contribute to the formation of an excess boron phase which may precipitate as a low melting eutectic at the austenitic grain boundaries. High boron contents, that is, in excess of about 0.060%, may also form excess boron phases which seriously affect the transverse ductility of the alloy. This precipitate being a low melting eutectic may seriously affect the hot workability of the alloy. The optimum combination of mechanical properties is obtained when the boron content is maintained within the range between about 0.003% and about 0.015%.

The titanium is highly critical to the alloy of this invention. It is present within the solid solution and substantially contributes to the strength of the alloy. It also contributes to the precipitation of the major strengthening precipitates which are transition phases of intermetallic compounds having the chemical formulae NlgTi and Nig (Al, Ti). When precipitated as a coherent precipitate, it produces an outstanding increase in the mechanical properties of the alloy. At least 2.0% titanium is necessary in order to obtain suflicient strengthening through the precipitation hardening reaction. Titanium contents in excess of about 6.0% do not appear to provide any further increase in the mechanical characteristics of this alloy and in addition, titanium contents beyond this amount form complex phases which are extremely deleterious to the mechanical properties. The optimum combination of mechanical properties is obtained when the titanium content is maintained within the range between 2.5% and about 3.5%. The balance of the alloy consists predominantly of iron with not more than 1.5% of incidental impurities, such as nitrogen, phosphorus, sulfur, copper and other impurities normally found in the commercial production of such alloys.

The alloy of this invention can be heat treated to obtain the optimum combination between creep rupture properties and tensile properties commensurate with good ductility, or the heat treatment can be varied in order to accentuate either the creep rupture properties or the tensile properties depending upon the desired characteristic in the end product. In order to obtain the optimum creep rupture properties, it is preferred to solution heat treat the alloy of this invention by heating it to a temperature in the range between 1900 F. and 2000 F. for a time eriod of about 1 hour and thereafter quench the alloy in oil. The alloy is then subsequently aged at a fimperature in the range of 1300" F. and 1375 F. for 'a time period of about 16 hours and thereafter air cooled. On the other hand, if the optimum tensile properties are desired within the alloy, it is preferred to solution heat treat the alloy at a temperature in the range between 1600 F. and 1700 F. for a time period of about 4 hours followed by a rapid quench usually in oiL' Thereafter the alloy is aged at a temperature between 1300 F. and 1375 F. for a time period of 16 hours and thereafter air cooled. Where the optimum combination between creep rupture properties and tensile properties is desired within the alloy, it is preferred to heat treat the alloy at a temperature within the range between 1750 II. It is to be noted that the stress rupture test is the one commonly used to evaluate the elevated temperature properties of alloys and consists of subjecting the alloy to a static load at a given temperature and measuring the time to cause rupture as well as the rate of the elongation of the alloy over a given period of time. Table III contains the data for both the smooth-bar rupture test and the notch-bar rupture test, the latter of which is performed under the same conditions as the standard smooth-bar rupture test, the only diiference being that the test specimen is provided with a machined V-notch at the reduced section which has a 0.005 inch radius at the root of the notch.

TABLE III Stress Rupture Properzies Hardness Temp. Stress El. RA. Heat No. Heat treatment Test (F.) .02? Hours (Peg (Peg)- S1 A C811 C911 0. ge

BHN BHN UMV-6 1,8001-0i1+1,3251G-AC 28 1 149 311 sa 11200 001000 s Univ-1034-.1,s00-1-0n+1,32516-Ao- NR 1,200 00,000 SR 1,200 75,000 SR 1,200 00,000 Univ-1044..1,s00-1-0i1+1,s2s-10-AO. NR ,200 00,000 s12 1,200 75, 000 D-975V- 1,8001-oi1+1,325-16-AO v-ss 1,9502-0i1+1,32516-AC v-ss -1.950-2-0i1+1,32516-AO 888 fii588 2?;888 1 981 Fi ff? 1 SR= Stress rupture, smooth bar; N R =S'tress rupture, notched bar.

F. and 1850 F. for a time period of about 2 hours followed by a rapid quench in oil. Thereafter the alloy may be aged at a temperature in the range between 1300 F. and 1375 F. for a time period of 16 hours and thereafter air cooled.

In addition to the above described heat treatments, it is preferred to homogenize the alloy in ingot form prior to hot working of any nature. The homogenization is preferably performed at a temperature within the range between 2150 F. and 2200 F. for a time period which may vary between 4 to 6 hours for very small ingots to time periods of up to 72 hours or longer for extremely large ingots. The homogenization heat treatment is effec tive for removing large amounts of M Ti phase which may form during the solidification of the ingot. The homogenization heat treatment is preferred to be performed, as a matter of course, during the normal commercial processing of this alloy.

In order to more clearly illustrate the advantages of this invention, a number of heats were made and tested to illustrate the elfect of titanium, manganese and silicon on the mechanical properties of the alloy of this invention. Reference is directed to Table II which illustrates the chemical composition of seven heats which will be referred to more fully hereinafter.

Reference is directed to Table III which contains the results of various tests performed on the alloys of Table TABLE II Chemical Composition (Percent by Weight) From the test results recorded in Table III, the elfect of silicon on the ductility of this alloy is clearly illustrated by comparing Heats UNIV-6, UMV-1034 and UMV-1044. Heat UMV-6 has an analysis of a commercially available alloy, and compared with UMV-1034 it is seen that decreasing the silicon from about 1.0% to about 0.03% is effective for increasing the smooth-bar rupture life, but at the expense of the ductility of the alloy and the notch sensitivity, that is, the ratio of the notch-bar rupture life to the smooth-bar rupture life. As was pointed out hereinbefore, the use of higher titanium contents is effective for producing greater rupture life. This fact is proved by comparing Heat UNIV-1044 with Heats UMV-6 and UNIV-1034 which effectively illustrate that the rupture life is increased from 671 hours for Heat UMV-1034 to 890 hours in Heat UNIV-1044 when the titanium content is increased from about 2.2% to about 4.3%, the balance of the elements remaining substantially the same. Here the effect of low silicon on the rupture ductility is clearly illustrated and shows that the alloy is practically brittle as illustrated by an elongation of only 0.3 If the alloy of Heat D-975V is compared with Heat UMV-1044, it is seen that a great increase in ductility is noted where the silicon content has been raised from 0.04% for Heat UMV-l044 to 0.78% for Heat D-975V. Substantially similar results were obtained in alloys V-SS, V-89 and V-90. It is at once apparent that the higher titanium contents are efiective for increasing the rupture life and Heat N o 0 Mn Si Cr Ni Mo Ti Al V B Fe 05 1. 28 1. 08 14. 45 25.07 1. 26 2.08 23 .29 Ba]. .04 1. 27 03 14.68 25. 05 1. 22 2. 20 05 35 Bill.

02 1. 04 .04 14. 30 25. 67 1. 30 4. 32 05 35 B81. .04 1. 14 .78 14. 94 26. 24 1. 33 4. 30 21 32 B21. .03 .09 01 14. 03 25. 79 1. 23 2. 35 11 24 Bal. .05 1.01 .02 15. 04 25. 96 1.21 2. 25 14 27 Hal. .06 1.58 8 14. 11 25.18 1.18 2. 48 .10 29 Bal.

1 Less than 0.006% B.

7 that extremely low silicon contents seriously decrease the ductility of the alloy.

In order to more clearly show the elfect of silicon at various titanium levels, a series of heats were made having various silicon contents at a given level of titanium. Manganese content was held below the detectable level by normal chemical analysis. The chemical analysis of these heats is set forth hereinafter in Table IV.

no appreciable effect upon the percentage elongation or reduction of area. Comparing these three alloys with alloys V-057A, V057C and V057E which have a corresponding silicon content ranging between 0.14% and 1.10% and a titanium content of about 3.0%, it is seen TABLE IV Chemical Composition (Percent by Wezght) Heat No Mn Si Or N1 M0 Ti Al None .02 13. 98 23. 66 1. 21 1. 95 .09 None .28 13. 94 24.24 1.18 1. 98 .09 None .66 13. 94 24.10 1.18 2. 03 .08 None 1.18 13.98 24.36 1. 1. 93 .08 None 1. 72 13. 94 24. 49 1. 20 2.12 .10 None 14 14.70 25. 39 1. 10 2. 90 08 None .20 14.68 23. 1.10 2. 89 .08 None .58 14. 08 26. 36 1.10 2. 94 10 None 1. 58 14.40 20. 24 1.11 2. 94 12 None 1.10 14.44 26. 20 1.10 2. 94 10 None 10 14. 90 24. 1. 22 3. 10 None .24 14. 86 24.78 1. 24 3. 5s 10 None 62 14.64 24. 94 1. 24 3. 52 .09

1 Boron analysis made on master heat and averaged less than 0, 005% The alloys as set forth in Table IV were subjected to tensile tests, both at room temperature and at 1200 F. It is to be noted also that the alloys of Table IV were also subjected to different solution heat treatments, and the test results recorded hereinafter in Table V also illustrate the effect of the dilferent solution heat treatments on the properties of these alloys.

TABLE V that substantially higher room temperature tensile properties are obtained; that is, the hardness, yield strength and tensile strength are increased with no appreciable efiect V i B Fe .18 Hal.

.21 Bal .18 Bal.

.20 Bal.

.27 Bal.

.25 Bal.

.26 Bal.

being shown upon the ductility of these alloys. However, increasing the silicon content appears to detract somewhat 90 from the tensile properties. These test results clearly Tensile Properties A. ROOM TEMPERATURE illustrate that the lower solution heat treatment temperature produces substantially higher tensile properties in these alloys than those tensile properties obtained using a solution heat treatment temperature of 1950 F. The

Soln. temp. 1,650 F. S0111. temp. 1,950 F.

Heat Hard 0.02% 0.2% urns. Elong. Red. of Hard 0.02% 0.2% U.T.S. Elong. 11941.01

(DPH) Y.S. Y.S. (p.s.i.) 5d, area, (DPH) Y.S. Y.S. (p.s.i.) 5d, area,

(p.s.1 (p.s.i.) percent percent (p.s.i.) (p.s i percent percent Referring more particularly to the test results recorded in Table V which illustrate the effect of varying he silicon content within these alloys at different levels of titanium, it is at once seen by comparing the alloys which were solution heat treated at 1650 F. with the same alloys which were solution heat treated at 1950 F. that the lower solution heat treatment temperature is preferred in order to obtain outstanding tensile properties, both at room temperature and at 1200" F. Specifically, the test results recorded in Section A reveal that with a titanium content of about 2% as in alloys V056A, V-056C and V-056E, increasing the silicon content from 0.02% up to 1.72% is effective for decreasing the room temperature yield strength, hardness and ultimate tensile strength with alloys of heats V-05 8A and V058C which have a titanium content of about 3.5% and corresponding silicon contents of 0.10% and 0.62%, respectively, again clearly illustrate that the higher titanium contents are effective for producing substantially higher tensile properties. At the lower solution heat treatment temperature, a slight increase in the percentage reduction of area is noted with the higher silicon contents. However, this trend appears to be reversed with the use of higher solution heat treatment temperatures.

Referring now more particularly to Section B of Table V which contains the test results recorded for the tensile tests performed at 1200 F., it is seen that substantially similar results were obtained as respects hardness, yield strength and tensile strength. However, the higher silicon contents appear to produce better ductility in these alloys as measured by the percentage reduction of area as Well as the percentage elongation, both with solution heat treatments at 1650" F. and at 1950 F. It should also be noted that the alloy of this invention maintains excellent tensile strength, yield strength, and hardness as well as ductility even when heated to the elevated temperatures of 1200 F. for testing.

1 the rate of creep and the rupture life is increased until the silicon content is beyond the range set forth hereinbefore in Table I, at which point the silicon content is efiective for decreasing the rupture life of the alloy. Again the higher silicon contents are efiective for producing higher percentages of elongation in this alloy. Heats V058-A, f V058B and V058C again illustrate the same eifect of silicon upon the rupture life, the ductility and the creep rate at a titanium level of 3.5 As would be expected,

When these same alloys, as set forth hereinbefore in the higher solution heat treatment temperature is efifective Table IV, were subjected to the creep rupture test, the effect of the heat treatment was reversed compared to what it was for the alloys as tested by the tensile test. Reference is directed to Table VI which illustrates the effect of varying the silicon content at different levels of 1 TABLE VI for producing outstanding rupture life in this alloy.

Referring now to Section B of Table VI and the test results recorded therein, the elfect of silicon, titanium and solution heat treatment temperatures on the notchr rupture sensitivity of these alloys is illustrated. From Subsection B it is seen that the higher silicon contents at the 2.0% titanium level decrease the rupture life for the Stress Rupture Properties (1200 F.)

A. STRESS 65,000 P.S.I.

Soln. temp. 1650" F. Soln. temp. 1950 F.

Heat N0. Rupture life (hrs.) Elong. Red. of Creep Rupture life (hrs) Elong. Red. of Creep 4d area rate 4d area rate (per- (perpercent] (per- (perercent/ V-notch Smooth cent) cent) hr. V-notch Smooth cent) cent) hr.

V-056A- 208 7. 3 9. 0 127 5. 5 7. 5 0.0110 B 133 6. 9 11.9 150 3. 3 6. 5 0.0110 131 5. 7 12. 9 92 6. 2 10.0 0.0100 33 29. 4 42. 4 41 11.9 20. 4 0. 1320 41 25. 3 61. 8 213 4. 8 5. 0 780 1.1 6 0 0. 0008 460 3. 1 3.6 1, 452 1. 1 3.0 0. 0012 324 6. 5 14.4 1, 114 2. 0 2.0 0. 0018 77 25. 4 64. 4 332 12. 8 29. 8 0. 0069 110 20. 8 48. 0 506 0.0050 103 2. 5 6. 1 1, 125 0.4 3. 0 0. 0007 119 6. 0 9. 5 1, 425 2. 1 3. 0 0.0016 375 6. 1 12.0 921 1. 5 2. 5 0. 0044 B. STRESS 80,000 P.S.I.

7. 5 9. 1 8. 9 8. 27. 8 39. 6 11.25 1.4 7. 4 27 2.0 8. 0 27. 5 2. 4 3. 7 47 7. 5 l7. 5 39 4.8 11. 1 100 1. 2 8. 6 V058B 74 1.4 9.8 17-0580. 1 233 2. 8 5.0

Referring more particularly to the test results recorded in Table VI and specifically to Heats V-056A, V-056B, V-056C, V-056D and V-056E which have a titanium content of about 2.0% and a silicon content varying between 0.02% and 1.72%, it is immediately seen, for the alloys solution heat treated at 1650" F. and increase the rupture life for the alloys solution heat treated at 1950 F. An increase in the level of the rupture life of the alloys with increasing silicon contents is noted when the titanium content is increased to 3.0%; however, no definite alloys solution heat treated at 1650 E, that increasing trend is ascertainable as respects the effect of silicon.

the silicon content decreases the rupture life of the alloy. The creep rate, while excellent, shows no particular trend by a variation in the silicon content at this low level of titanium. However, the ductility is markedly increased with increasing silcon. Where the alloys are solution heat treated at 1950 F., it would appear that the higher silicon contents are effective for increasing the ductility at the expense of both the creep rate and the rupture life. Heats V-056B and V-056C appear to have an excellent creep rate with adequate rupture life and fair ductility. If these heats are compared with Heats V-057A, V-057B, V-057C, V-057D and V-057E, a clearer picture of the effect of silicon at the 3% titanium level is shown. When ture life is usually greater than one.

the greatest mechanical properties are obtained when the titanium content is in the range between about 2.0% and about 6.0% and preferably within the range between 2.5% and 3.5%.

In order to more clearly illustrate the effect of both these alloys are solution heat treated at 16500 it is manganese and silicon on the alloys of this invention, refseen that increasing the silicon content produces a steady increase in the ductility of the alloy with a corresponding reduction in the rupture life as well as an increase in the rate of creep. However, if these same alloys are heat erence is directed to Table VII which contains a series of alloys having a titanium content of about 3.2% and varying silicon contents at predetermined levels of manganese. Table VII records the chemical analysis of each of these treated at 1950" F., there is an outstanding decrease in heats.

by Weight) Heat No. i Mn i Si 1 Cr Ni Mo Ti Al I V B BV-371 042 01 60 14. 92 24. 98 1. 43 3. 3O 26 001 BV-372. 038 09 42 15. 00 24. 90 1. 41 3. 20 28 007 BV373 07 60 14. 64 25. 10 l. 38 3. 20 31 26 006 BV-374. 09 74 14. 88 25. 06 1. 38 3. 20 32 27 007 BV-376- 22 62 15. 48 24. 94 1. 42 3. 20 30 27 006 BV-377. 22 76 15. 08 24. 94 1. 41 3. 20 34 29 005 BV-378 33 42 15. 28 24. 78 l. 41 3. 25 33 .27 006 BV379 35 62 15.28 24. 98 1. 38 3. 25 32 28 005 Reference is directed to Table VIII which illustrates the eiiect of the silicon on the tensile properties and the creep rupture properties for alloys of various predetermined manganese contents. In each case the alloy, as set forth in Table VIII, has been first subjected to a heat treatment consisting of a solution heat treatment at a tempertaure of 1800 F. for a time period of about 2 hours followed by an oil quench and thereafter the alloy was aged at a temperature of about 1350 F. for a time period of about 16 hours and thereafter air cooled.

TABLE VIII Mechanical Properties higher titanium contents to be used without the formation of undesirable amounts of M Ti phase and G-phase as set forth hereinbefore. In addition, there are no particular equipment, processes or skills necessary to practice this invention and the results obtained through the use of the alloy of this invention have been outstanding. The alloy of this invention is especially useful for gas turbine applications, for example, as turbine wheels and buckets, bolting and structural applications and can also be used as skin covering in missile applications and the like where Tensile properties 1,200 F Stress rupture properties, 1,300 F- 65,000 p.s.i.

Heat No.

002% 0.2% U.'I.S. El. Red. Rupture E1. Bed. Time for Y.S. Y.S. (p.s.i.) (pen area life (perarea 0.5% (p.s.i.) (p.s.i.) cent) (per- (hrS.) cent) (percreep cent) cent) (hrs.)

BV-371.-- 70, 500 101, 000 130, 800 26. 4 56.9 22 6. 5 9.6 6.0 BV-372 100,000 124, 800 136, 000 26. 6 47. 6 6. 6 11.9 11.2 BV-373--- 65, 700 97, 600 132, 800 36. 6 50. 3 29 7. 1 13.8 7. 5 BV374 85, 300 107, 000 134, 000 33. 6 51. 3 28 12. 5 17. 6 7. 6 BV376 82,200 107, 500 134,800 36. 6 57. 2 35 10. 1 12. 4 11.0 BV-377 93, 500 111,000 137, 800 34. 8 58. 5 25 11.4 19.0 3. 6 BV378 86, 300 106, 500 138, 000 33.4 56. 6 37 10.3 12. 9 8.0 BV379 82, 500 102,200 134, 200 32. 5 55. 5 25 11.9 18.3 3. 9 BV380 87, 000 108,700 137, 900 32. 5 56. 5 27 6. 5 14. 8 6. 2

Heats BV-372, BV-373 and BV-374 illustrate the effect of increasing the silicon content from about 0.42% to about 0.74% with a corresponding manganese content of about 0.09%. It is apparent that increasing the silicon content is effective for decreasing the tensile properties as well as the stress rupture properties. A corresponding increase in the ductility of these alloys is also apparent. It is clear, however, that the level of these properties is excellent compared with previously known alloys having high manganese and high silicon contents. When the manganese content is raised to about 0.20% for Heats BV-376 and BV-377, substantially similar results are recorded for the stress rupture properties with no appreciable difference appearing in the tensile properties. At higher manganese contents, that is, at about 0.35%, the same trend is also witnessed. It is thus apparent that excellent results are obtained where the manganese is maintained below a maximum of 0.50%, and preferably within the range between 0.05% and 0.35%. It is also noteworthy to point out that boron has a definite effect upon the alloy of this invention. Comparing Heats BV-376 and BV-377 with BV-37l, the latter having less than 0.001% boron therein, it is seen that an increase is noted in both the tensile properties and the stress rupture properties in the alloy of this invention when boron is present. In addition to the normal effect, boron also suppresses the formation of undesirable lamellar precipitates which can be formed during various combinations of heat treatments and working procedures.

From the foregoing, it is apparent that the alloy of this invention produces an outstanding result. This is achieved only through the proper relation of the manganese and silicon contents which cooperate to permit the elevated temperatures of up to at least 1300 F. are encountered.

I claim:

1. An austenitic iron-base alloy having a composition within the range between about 0.01% and about 0.15% carbon, between about 0.05% and 0.80% silicon, traces to about 0.50% manganese, between about 12.0% and about 22.0% chromium, between about 15.0% and 35.0% nickel, between about 0.25% and about 5.0% molybdenum, between about 2.0% and about 6.0% titanium, traces to about 1.25% aluminum, between about 0.10% and 1.5% vanadium, between about 0.001% and 0.060% boron, and the balance iron with incidental impurities.

2. An austenitic iron-base alloy having a composition within the range between about 0.01% and about 0.08% carbon, between about 0.35% and 0.70% silicon, 0.05% to about 0.35% manganese, between about 13.5% and about 16.0% chromium, between about 24.0% and 27.0% nickel, between about 1.0% and about 1.5% molybdenum, between about 2.5% and about 3.5% titanium, 0.10% to about 0.50% aluminum, between about 0.10% and 0.50% vanadium, between about 0.003% and 0.015% boron, and the balance iron with incidental impurities.

3. An austenitic iron-base allow having a composition of about 0.017% carbon, less than 0.05% manganese, about 0.58% silicon, about 14.68% chromium, about 26.36% nickel, about 1.10% molybdenum, about 2.94% titanium, about 0.10% aluminum, about 0.26% vanadium, 0.002% boron, and the balance iron with incidental impurities.

4. An article of manufacture for use in highly stressed 13 parts operating at elevated temperatures of up to at least 1300 F. and higher, formed from an austenitic ironbase alloy consisting of from 0.01% to 0.15% carbon, from 0.05% to 0.80% silicon, from traces to 0.50% manganese, from 12.0% to 22.0% chromium, from 15.0% to 35.0% nickel, from 0.25% to 5.0% molybdenum, from 2.0% to 6.0% titanium, from traces to 1.25% aluminum, from 0.10% to 1.5% vanadium, from 0.001% to 0.060% boron, and the balance iron with incidental impurities.

5. An article of manufacture for use in highly stressed parts operating at elevated temperatures of up to at least 1300 F. and higher, formed from an austenitic ironbase alloy consisting of from 0.01% to 0.08% carbon, from 0.35% to 0.70% silicon, from 0.05% to 0.35% manganese, from 13.5% to 16.0% chromium, from 24.0% to 27.0% nickel, from 1.0% to 1.5% molybdenum, from 2.5% to 3.5% titanium, from 0.10% to 0.50% aluminum, from 0.10% to 0.50% vanadium, from 0.003% to 0.015% boron, and the balance iron with incidental impurities.

6. A precipitation hardened article of manufacture suitable for use under high stress at temperatures of up to at least 1300 F. having optimum combination of creep strength and ductility together with tensile properties and formed from an austenitic iron-base alloy consisting of from 0.01% to 0.15% carbon, from 0.05% to 0.80% silicon, from traces to 0.50% manganese, from 12.0% to 22.0% chromium, from 15.0% to 35.0% nickel, from 0.25% to 5.0% molybdenum, from 2.0% to 6.0% titanium, from traces to 1.25% aluminum, from 0.10% to 1.5 vanadium, from 0.001% to 0.060% boron, and the balance iron, and which has been given a homogenizing heat treatment while in ingot form by heating to a temperature in the range between 2150 F. and 2200 F. for a time period in the range between 2 hours and 72 hours and thereafter air cooled, and which has been quenched from a solution heat treatment at a temperature in the range between 1750 F. and 1850 F. for a time period ranging between /2 hour and 4 hours and thereafter aged at a temperature in the range between 1250 F. and 1400 F. for a time period ranging between 4 hours and- 48 hours followed by air cooling to room temperature.

7. A precipitation hardened article of manufacture suitable for use under high Stress at temperatures of up to at least 1300 F. having optimum combination of creep strength and ductility together with tensile properties and formed from an austenitic iron-base alloy consisting of from 0.01% to 0.08% carbon, from 0.35% to 0.70% silicon, from 0.05% to 0.35% manganese, from 13.5% to 16.0% chromium, from 24.0% to 27.0% nickel, from 1.0% to 1.5% molybdenum, from 2.5% to 3.5% titanium, from 0.10% to 0.50% aluminum, from 0.10% to 0.50% vanadium, from 0.003% to 0.015 boron, and the balance iron, and which has been given a homogenizing heat treatment while in ingot form by heating to a temperature in the range between 2150 F. and 2200 F. for a time period ranging between 2 hours and 72 hours and thereafter air cooled, and which has been quenched from a solution heat treatment at a temperature in the range between 1750 F. and 1850 F. for a time period ranging between A2 hour and 4 hours and thereafter aged at a temperature in the range between 1250 F. and 1400 F. for a time period ranging between 4 hours and 48 hours followed by air cooling to room temperature.

8. A precipitation hardened article of manufacture suitable for use under high stress at a temperature of up to at least 1300 F. having optimum tensile strength and ductility and formed of an austenitic iron-base alloy consisting of from 0.01% to 0.15% carbon, from 0.05% to 0.80% silicon, from traces to 0.50% manganese, from 12.0% to 22.0% chromium, from 15.0% to 35.0% nickel, from 0.25% to 5.0% molybdenum, from 2.0% to 6.0%

titanium, from traces to 1.25% aluminum, from 0.10% to 1.5% vanadium, from 0.001% to 0.060% boron, and the balance iron, and which has been given a homogenizing heat treatment while in ingot form by heating to a temperature in the range between 2150 F. and 2200 F. for a time period ranging between 2 hours and 72 hours and thereafter air cooled, and which has been quenched from a solution heat treatment at a temperature in the range between 1600 F. and 1700 F. for a time period ranging between 2 hours and 8 hours and thereafter aged at a temperature in the range between 1250 F. and 1400 F. for a time period ranging between 4 hours and 48 hours followed by air cooling to room temperature.

9. A precipitation hardened article of manufacture suitable for use under high stress at a temperature of up to at least 1300 F. having optimum tensile strength and ductility and formed of an austenitic iron-base alloy consisting of from 0.01% to 0.15% carbon, from 0.05 to 0.80% silicon, from traces to 0.50% manganese, from 12.0% to 22.0% chromium, from 15.0% to 35.0% nickel, from 0.25% to 5.0% molybdenum, from 2.0% to 6.0% titanium, from traces to 1.25 aluminum, from 0.10% to 1.5% vanadium, from 0.001% to 0.060% boron, and the balance iron, and which has been quenched from a solution heat treatment at a temperature in the range between 1600 F. and 1700" F. for a time period ranging between 2 hours and 8 hours and thereafter aged at a temperature in the range between 1250 F. and 1400 F. for a time period ranging between 4 hours and 48 hours followed by air cooling to room temperature.

10. A precipitation hardened article of manufacture suitable for use under high stress at a temperature of up to at least 1300 F. having optimum creep strength and ductility and formed of an austenitic iron-base alloy consisting of from 0.01% to 0.15% carbon, from 0.05% to 0.80% silicon, from traces to 0.50% manganese, from 12.0% to 22.0% chromium, from 15.0% to 35.0% nickel, from 0.25% to 5.0% molybdenum, from 2.0% to 6.0% titanium, from traces to 1.25% aluminum, from 0.10% to 1.5% vanadium, from 0.001% to 0.060% boron, and the balance iron, and which has been given a homogenizing heat treatment while in ingot form by heating to a temperature in the range between 2150 F. and 2200 F. for a time period ranging between 2 hours and 72 hours and thereafter air cooled, and which has been quenched from a solution heat treatment at a temperature in the range between 1900 F. to 2000 F. for a time period ranging between /2 hour and 4 hours and thereafter aged at a temperature in the range between 1250 F. and 1400 F. for a time period ranging between 4 hours and 48 hours followed by air cooling to room temperature.

11. A precipitation hardened article of manufacture suitable for use under high stress at a temperature of up to at least 1300 F. having optimum creep strength and ductility and formed of an austenitic iron-base alloy consisting of from 0.01% to 0.15% carbon, from 0.05% to 0.80% silicon, from traces to 0.50% manganese, from 12.0% to 22.0% chromium, from 15.0% to 35.0% nickel, from 0.25% to 5.0% molybdenum, from 2.0% to 6.0% titanium, from traces to 1.25% aluminum, from 0.10% to 1.5% vanadium, from 0.001% to 0.060% boron, and the balance iron, and which has been quenched from a solution heat treatment at a temperature in the range between 1900 F. to 2000 F. for a time period ranging between /2 hour and 4 hours and thereafter aged at a temperature in the range between 1250 F. and 1400 F. for a time period ranging between 4 hours and 48 hours followed by air cooling to room temperature.

12. A precipitation hardened article of manufacture suitable for use under high stress at temperatures of up to at least 1300 F. having optimum combination of creep strength and ductility together with tensile properties and formed from an austenitic iron-base alloy having a composition consisting of from 0.01% to 0.15% carbon, from traces to 0.50% manganese, from 0.05% to 080% silicon, from 12.0% to 22.0% chromium, from 15.0% to 35.0% nickel, from 0.25% to 5.0% molybdenum, from 2.0% to 6.0% titanium, traces to 1.25% aluminum, from 0.10% to 1.5% vanadium, from 0.001% to 0.060% boron, and the balance iron, and which has been quenched from a solution heat treatment at a temperature in the range between about 1750 F. and about 1850 F. for a time period between 1 hour and 4 hours and thereafter followed by an aging treatment at a temperature in the range between about 1250 F. and 1400 F. for a time period between 4 hours and 48 hours and thereafter air cooled.

References fiited in the file of this patent 

1. AN AUSTENITIC IRON-BASE ALLOY HAVING A COMPOSITION WITHIN THE RANGE BETWEEN ABOUT 0.01% AND ABOUT O.15% CARBON, BETWEEN ABOUT 0.05% AND 0.80% SILICON, TRACES TO ABOUT 0.50% MANGANESE, BETWEEN ABOUT 12.0% AND ABOUT 22.0% CHROMIUM, BETWEEN ABOUT 15.0% AND 35.0% NICKEL, BETWEEN ABOUT 0.25% AND ABOUT 5.0% MOLYBDENUM, BETWEEN ABOUT 2.0% AND ABOUT 6.0% TITANIUM, TRACES TO ABOUT 1.25% ALUMINUM, BETWEEN ABOUT 0.10% AND 1.5% VANADIUM, BETWEEN ABOUT 0.001% AND 0.060% BORON, AND THE BALANCE IRON WITH INCIDENTAL IMPURITIES. 